by McDave » Wed Dec 21, 2016 10:59 pm
What flboy says is true and valid. However, there are times when the light, fan or other 12v device is not located at or near a chassis ground. It's no more difficult to pull a 2 conductor,(black/wht or red/ blk etc.) to the device location, than to pull a hot, and then a wire to chassis. The real advantage comes when you may have to troubleshoot. You don't have to look for the chassis grnd. Ground and positive are located at the same place at fixture, and same place at origin.
If you are diagnosing a circuit and don't have 12v present, you still wont know if you are missing positive or ground until you get to chassis grnd. If both sides of wire go to same place, you can check them there.
Chassis grounds can also add resistance and therefore cause voltage drop, but that is rarely a problem. The simple mechanics of making a solid chassis ground, ring terminal screwed to a clean spot of chassis, tightly crimped to wire,
then connected to grnd wire at light etc., is really more labor and more connections to fail than to have two connections at each end of wire.
I mostly use telegraph or "Western Union" connections at the light. The wires are wrapped together to provide a mechanical connection, then soldered to provide the electrical connection, then shrink tube to provide insulation and strain relief. Crimped on "butt" connectors can be problematic and scotch locks are almost guaranteed to fail at some point.
Since you will be making these connections, they will have your name on them. Can't really blame "Made in China" for a failure. It's not that much harder to make a good, reliable connection than to make lesser one or two or three. At the end of the day when you close your eyes, you aren't worried about those conections, you made them, you know they are right. But there are lots and lots of others to keep you awake. I'm not sure if Wang Fung cares about his work as much as you do.
McDave